1. Field of the Invention
The present invention relates to the field of multiplexed data encoders and decoders and in particular to such encoders and decoders as used in test flight recorders.
2. Description of the Prior Art
The prior art telemetry or test flight equipment, which is used to record multiplexed data on a military-standard bus (1553 MUX) generates enough information within approximately two minutes of recording time to fill a thirteen-track one- or two-inch magnetic tape which is driven at 120 inches per second. High tape speeds and fidelity is required to record the data inasmuch as the data is extremely broadband, ranging from DC to 1 MHz. Normally, the data is telemetered via a video broadcaster to a field receiving unit. Typically, the transmission of such video telemetered data is line of sight. However, broadcast telemetry is inherently limited by loss of information wherever ground or air transmission is temporarily blocked by an optical or other electromagnetic obstruction or interference.
The data may also be recorded on magnetic tape in a test flight recorder which is subsequently transported to a data processing center. However, such broadband data recordal on tape is difficult under field conditions and is error prone. There has been considerable prior art effort to reduce the effective frequency recorded on the magnetic tape is a manner so that errors are not introduced or data is not lost.
One prior art method is to put data from a multiplexed bus onto two tracks of a recording tape which is driven at 30 inches per second (ips). When the data is thus divided between two tracks, it then must be carefully reassembled to reconstruct the actual multiplexed bus data. This process is also difficult and extremely error prone. As a practical matter, only 80-85% of the data can be accurately reconstructed when a 1-MHz Manchester code is thus split into two tracks.
It is known in the art to convert digital coding in one format, such as NRZ code into other specialized formats for the purposes of hardwire transmission or intermediate storage on tape. Examples of such conversion can be found in Frazier, Jr., "System for Transmission, Storage and/or Multiplexing of Information", U.S. Pat. No. 3,723,982 (1973); and Lacher, "Bipolar Time Modulated Encoder/Decoder System", U.S. Pat. No. 4,503,472 (1985). In particular, it is well understood that Manchester or other biphase coding can be converted into NRZ formats and then the data in NRZ format processed for the purpose of signal conditioning or data processing. Examples of this conversion and use is shown in Perkins et al., "Data Converting and Clock Pulse Generating System", U.S. Pat. No. 3,659,286 (1972); Norris, "Split Phase Adaptive Decoding Electronics", U.S. Pat. No. 3,646,546 (1972); and Kostenbauer et al., "Digital Formal Converter", U.S. Pat. No. 3,705,398 (1972).
It is also recognized in the art that certain coding formats are recognized as having a higher data bit packing density on magnetic tape than Manchester codes. For example, in Miller, "Recording and/or Reproduction System", U.S. Pat. No. 3,108,261 (1963), the DM-M code is described as having such a higher data bit packing density.
The particular problem is that in the Military Standard 1553 MUX the data word is characterized by a singularly defined format. For example, in Military Standard 1553 the word format is a Manchester-2 biphase level. A logical 1 is transmitted as a bipolar coded signal, 1/0, that is a positive pulse followed by a negative pulse. A logical zero is coded as the signal, 0/1, that is a negative pulse followed by a positive pulse. A transition through the zero voltage level occurs at the midpoint of each bit interval. In the 1553 Mux the digital transmission rate is at one megabit per second. Each word is 16 bits concatentated with a synchronization waveform prefix and a suffix parity bit suffix for a total of 20 bit time intervals. The synchronization waveform is a three-bit interval, invalid Manchester pattern. This synchronization waveform, which is an invalid Manchester code, may have different formats in different types of words depending upon whether the Military Standard 1553A or 1553B is used.
In all prior art applications where the conversion from one code to another is considered and in particular when a conversion from Manchester Code to NRZ or to another code is considered, it is assumed that there exists a uniform methodology for converting each portion of each word. Therefore under prior art methods, if Manchester Code is to be encoded into an intermediary code format for recordal on tape, the synchronization information will be lost or garbled.
Therefore, what is needed is some means whereby data generated at a 1-MHz date rate, such as Manchester code, can be converted when used in word formats having invalid code sections, such as in the Military Standard 1553, to a more compact code density for tape recordal. Following tape recordal, then, additional means and methodologies are required to decode the data from the intermediary code back into Manchester coding such that the invalid synchronization prefix is appropriately reassembled.